The 5 Kinds of Nucleotides

Each is a polymer made up of 3 parts

Five nucleotides are commonly used in biochemistry and genetics. Each nucleotide is a polymer made up of three parts:

  • A five-carbon sugar (2'-deoxyribose in DNA or ribose in RNA)
  • A phosphate molecule
  • A nitrogenous (nitrogen-containing) base

Names of Nucleotides

In DNA, there are four nucleotides: adenine, thymine, guanine, and cytosine. Uracil replaces thymine in RNA.
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The five bases are adenine, guanine, cytosine, thymine, and uracil, which have the symbols A, G, C, T, and U, respectively. The name of the base is generally used as the name of the nucleotide, although this is technically incorrect. The bases combine with the sugar to make the nucleotides adenosine, guanosine, cytidine, thymidine, and uridine.

Nucleotides are named based on the number of phosphate residues they contain. For example, a nucleotide that has an adenine base and three phosphate residues would be named adenosine triphosphate (ATP). If the nucleotide has two phosphates, it would be adenosine diphosphate (ADP). If there is a single phosphate, the nucleotide is adenosine monophosphate (AMP).

More Than 5 Nucleotides

Although most people learn only the five main types of nucleotides, there are others, including, for example, cyclic nucleotides (e.g., 3'-5'-cyclic GMP and cyclic AMP). The bases can also be methylated to form different molecules.

How the Parts of a Nucleotide Are Connected

The parts of a nucleotide are a nucleoside plus one or more phosphate groups.
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Both DNA and RNA use four bases, but they don't use the same ones. DNA uses adenine, thymine, guanine, and cytosine, while RNA uses adenine, guanine, and cytosine but has uracil instead of thymine. The helix of the molecules forms when two complementary bases form hydrogen bonds with each other. Adenine binds with thymine (A-T) in DNA and with uracil in RNA (A-U). Guanine and cytosine complement each other (G-C).

To form a nucleotide, a base connects to the first or primary carbon of ribose or deoxyribose. The number 5 carbon of the sugar connects to the oxygen of the phosphate group. In DNA or RNA molecules, a phosphate from one nucleotide forms a phosphodiester bond with the number 3 carbon in the next nucleotide sugar.

Adenine Base

Adenine molecule, where gray atoms are carbon, white are hydrogen, and blue are nitrogen.
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The bases take one of two forms. Purines consist of a double ring in which a 5-atom ring connects to a 6-atom ring. Pyrimidines are single 6-atom rings.

The purines are adenine and guanine. The pyrimidines are cytosine, thymine, and uracil.

The chemical formula of adenine is C5H5N5. Adenine (A) binds to thymine (T) or uracil (U). It's an important base because it's used not only in DNA and RNA, but also for the energy carrier molecule ATP, the cofactor flavin adenine dinucleotide, and the cofactor nicotinamide adenine dincucleotide (NAD).

Adenine vs. Adenosine

Remember, although people tend to refer to the nucleotides by the names of their bases, adenine and adenosine aren't the same thing. Adenine is the name of the purine base. Adenosine is the larger nucleotide molecule made up of adenine, ribose or deoxyribose, and one or more phosphate groups.

Thymine Base

Thymine molecule, where gray atoms are carbon, white are hydrogen, red are oxygen, and blue are nitrogen.
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The chemical formula of the pyrimidine thymine is C5H6N2O2. Its symbol is T and it's found in DNA but not RNA.

Guanine Base

Guanine molecule, where gray atoms are carbon, white are hydrogen, red are oxygen, and blue are nitrogen.
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The chemical formula of the purine guanine is C5H5N5O. Guanine (G) binds only to cytosine (C), in both DNA and RNA.

Cytosine Base

Cytosine molecule, where gray atoms are carbon, white are hydrogen, red are oxygen, and blue are nitrogen.
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The chemical formula of the pyrimidine cytosine is C4H5N3O. Its symbol is C. This base is found in both DNA and RNA. Cytidine triphosphate (CTP) is an enzyme cofactor that can convert ADP to ATP.

Cytosine can spontaneously change into uracil. If the mutation is not repaired, this can leave a uracil residue in DNA.

Uracil Base

Uracil molecule, where gray atoms are carbon, white are hydrogen, red are oxygen, and blue are nitrogen.
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Uracil is a weak acid that has the chemical formula C4H4N2O2. Uracil (U) is found in RNA, where it binds with adenine (A). Uracil is the demethylated form of the base thymine. The molecule recycles itself through a set of phosphoribosyltransferase reactions.

One interesting factoid about uracil is that the Cassini mission to Saturn found that its moon Titan appears to have uracil on its surface.